Method for forging titanium alloy forging and forged titanium alloy material

ABSTRACT

The invention provides a titanium alloy having a narrow distribution of material properties in the thickness direction, allowing easy to finish the surface of the forged material after forging, during working the product to the final shape. The forged titanium alloy has a low sensitivity for cracking, excellent workability, and favorable ductility and fatigue properties, and provides a method for forging the titanium alloy. In order to attain the forged titanium alloy and the method for forging thereof, forging the titanium alloy, which has Tβ ° C. of the β-transus, is conducted, while keeping the relation of 400° C.≦Td and (Tβ−400)° C.≦Tm≦Tβ at a strain rate of a range from 2×10 −4  s −1  to 1 s −1 , further limiting the chemical composition of the titanium alloy, keeping the relation of [(Tm−Td)≦250° C.] during forging, where Tβ (° C.) is the β-transus of the titanium alloy, Tm(° C.) is the temperature of the work material for being forged, and Td(° C.) is the temperature of a die. The titanium alloy, which is produced by the above-described method, has a fine microstructure to be controlled for forming a specified microstructure, has a uniform and homogeneous material properties in the thickness direction, and has an excellent ductility and a fatigue properties.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for forging titaniumalloy, and also to a preferable titanium alloy forging stock and to apreferable forged titanium alloy.

[0003] 2. Description of the Related Art

[0004] Owing to the excellent material properties, titanium and titaniumalloy are widely used in chemical plants, power generators, medicalinstruments, and aircraft components. In particular, α+β type titaniumalloy has a light weight and has a high strength so that this type oftitanium alloy has widely been used in several fields. For instance, aturbine blade has a tendency to enlarge the size and to reduce theweight, in order to aim at the higher efficiency of facilities. In thisfield, the titanium alloy has been used. And, this type titanium alloyhas been utilized in aircraft components such as landing gears, whichrequest reducing the weight, taking the object of usage intoconsideration. Furthermore, the α+β type titanium alloy has beenutilized in movable machine parts, such as automobile parts includingconnecting rod and valve, and commercial goods such as golf club head.

[0005] However, generally speaking, titanium alloys have highsusceptibility to cracking, compared with steels, which are widely usedin the industries at present. And the hot deformation resistance oftitanium alloys is relatively higher at a low temperature range, so itis necessary to work at high temperature range for titanium alloys. Withregard to these characteristics, they are described in the “TITAN NOKAKO GIZYUTSU” published by Japan Titanium Society. In a hot workingprocess within high temperature range, especially about forging, thereare some technical issues such as surface oxidation and grain coarseningin higher temperature range, and cracking caused by brittle α-case whentemperature is down. On the contrary, in a working process within a lowtemperature range, high hot deformation resistance happens as one of thetechnical issue. Moreover, the temperature drops by contacting with atooling and consequent deterioration of workability also happen as oneof the technical issues. And it occurs a problem that inhomogeneousmicrostructure is formed by the adiabatic heat during working at thehigh strain rate.

[0006] As mentioned above, processing window of titanium alloy is verynarrow. Furthermore, in case of applying a conventional forging process,the resulted microstructure is different, in comparison with near thesurface area, where temperature drop is caused by contacting with die,and the mid-thickness portion where the temperature drops slowly or thetemperature increases by the adiabatic heat. And, particularly near thesurface layer, from time to time, working within a low temperature rangecauses elongated microstructure and the working within a low rangecauses increase of hardness. As a result, some sorts of problems are aptto happen, concerning the defective material properties.

[0007] However, from the viewpoint of manufacturing process, severaltimes of reheating and repeated forging are indispensable, due to thenarrow processing window of titanium. Furthermore, deterioration ofmaterial properties such as ductility and fatigue properties, which iscaused by grain coarsening, is also one problem, additionally tocomplicating the forging process. And there arise some sorts ofproblems, which means, finishing the oxidized surface should beindispensable after forging. Especially in case of dealing with acomplex shape of the forged products, taking into consideration that themicrostructure is changeable by reheating, the number of repetitioncycle to reheat and to forge should be limited. And the forgingindependently may not always attain a satisfactory requested finalshape. In that case, the finishing allowance increases the working loadincreases, and the yield of the charged material decreases. Furthermore,the oxidized scale and the deteriorated surface layer such as á-casesignificantly influence on the material properties, so it becomesnecessary to remove the deteriorated layer in the actual use of forging.In addition, in case that no satisfactory final shape is obtained,grinding is required to an excessive degree. That is to say, the narrowprocessing window and the grinding after hot working, bring out thehigher cost. Accordingly, concerning the production of the titaniumproducts, the working cost becomes higher, additional to the highermaterial cost.

[0008] In order to solve these problems as a concrete means, the forgingmethods, which means, spending more time and spending more labor, incomparison with the prior arts, are adopted in the present invention.One concrete method in the present invention is isothermal forging andhot die forging. In some cases in recent years, there has been describedin “Materials Properties Handbook Titanium Alloys”, “TITANIUMTECHNOLOGY”, and “TITANIUM AND TITANIUM ALLOYS”, published by ASM. Thesemethods adopt forging by heating not only the work material for forgingbut also the die. The work material and the die are heated to the degreeof the same temperature with that of the work material for forging. Or,elsewhere, the work material and the die are heated to the degree of thevery close temperature with that of the work material for forging.

[0009] And when this method are used, the strain rate is strictlycontrolled as low as at the degree of around 10⁻⁴ to 10⁻⁵ s⁻¹. Forexample, isothermal forging of Ti-6Al-4V alloy is done by selecting thetemperature of work material within an approximate range from 900° C. to950° C. And the temperature of die is also controlled within anapproximate range from 900° C. to 950° C. Also when the hot die isforged, the die temperature is controlled to the degree of anapproximate range, which is, from 650° C. to 800° C. The range is veryclose to the temperature of the work material. These methods make itpossible to suppress the temperature drop of work material. The methodsinvite the results in attaining favorable metal flow for obtaining aprecise shape by way of forging. Furthermore, the number of reheatingcycles decreases. The charged weight of the work material is saved.Additionally, the uniform microstructure through thickness can beobtained.

[0010] Since these methods depend mainly on working at a low strainrate, the forging load decreases to some extent. Furthermore, forging insuch an atmospheric condition, that is, the titanium is suppressed to beoxidized, for example, making use of an inert gas and making use of avacuum atmosphere, enable us to suppress the oxidation.

[0011] However, in these methods, the material is kept to be at a hightemperature for a long time, because the work material and the die havethe limitation to be heated, so that there happens a problem that thegrain is coarsened. In addition, the die is heated to a degree of hightemperature, whose temperature is as the same as the of the workmaterial. Elsewhere, the die temperature heated is very close to thetemperature of the work material. Therefore, the following kind of thedie needs to be adopted. For instance, an expensive Ni-base alloy isused, which is durable within a high temperature range, and, which hasexcellent heat resistance and oxidation resistance, as described in“Materials Properties Handbook Titanium Alloys” of ASM. Additionallyspeaking, there has a possibility to cause a problem such that theelectric discharge machinery is expensive, in order to work the die.Concerning the problem, it is easy to obtain a good metal flow by makinguse of the isothermal forging method and by the hot die forging method.However, the uppermost layer of the material, which gets contact withthe die, receives the friction by the die. And the difference happens inthe microstructure between the inner portion and the portion near thesurface area occurs about some kinds of titanium alloy.

SUMMARY OF THE INVENTION

[0012] The present invention provides a method for solving the problemsof material and for carrying out the manufacturing method. Concretelyspeaking, the object of the present invention is to provide a titaniumalloy that has less distribution of the material properties in thicknessdirection thereof. And another object of the present invention is toprovide the titanium alloy, which is requested to have fewer surfacesfinishing after forging. And the titanium alloy has low sensitivity forcracking, excellent workability, and favorable ductility and fatigueproperties. Simultaneously, the present invention is to provide afavorable forging stock and a method for forging.

[0013] Firstly, the present invention provides a method for forging atitanium alloy, which comprises:

[0014] preparing the titanium alloy as the forging stock;

[0015] forging the titanium alloy as the forging stock to have a workhardening factor, whose value is 1.2 or smaller, for obtaining a forgedtitanium alloy having a uniform material properties;

[0016] wherein the work hardening factor is defined as

work hardening factor=Hv(def)/Hv(ini)

[0017] wherein, Hv(ini) is the hardness of the titanium alloy as theforging stock before forging, and

[0018] Hv(def) is the hardness of the forged titanium alloy under thereduction of 20%.

[0019] Secondly, the present invention provides the method for forgingthe titanium alloy according to firstly mentioned method, wherein thedifference of the hardness between the thickness center portion of theforged titanium alloy and near the surface area of the forged titaniumalloy is 60 or less of Vickers hardness.

[0020] Thirdly, the present invention provides a method for forging atitanium alloy, which comprises:

[0021] preparing the titanium alloy as a forging stock;

[0022] forging the titanium alloy as forging stock, at strain rates from2×10⁻⁴ s⁻¹ to 1 s⁻¹ , while keeping the relation of (Tâ−400)° C.≦Tm≦900° C. and 400° C.≦Td≦700° C., to obtain a forged titanium alloyhaving a uniform material properties,

[0023] wherein, Tβ (° C.) is a β-transus of the titanium alloy,

[0024] Tm(° C.) is the temperature of the work material for forging, and

[0025] Td(° C.) is the temperature of a die.

[0026] Fourthly, the present invention provides the method for forgingthe titanium alloy according to the thirdly mentioned method, whereinthe temperature of the die, Td(° C.), and the temperature of the workmaterial for forging, Tm(° C.), are controlled to satisfy the relationof (Tm−Td)≦250° C.

[0027] Fifthly, the present invention provides the method for forgingthe titanium alloy according to the thirdly and fourthly mentionedmethods, wherein the titanium alloy as the forging stock contains Al: 4to 5%, V: 2.5 to 3.5%, Fe:1.5 to 2.5% , and Mo.: 1.5 to 2.5%, by masspercentage.

[0028] Sixthly, the present invention provides the method for forgingthe titanium alloy, according to the thirdly, fourthly and fifthlymentioned methods, wherein

[0029] a titanium alloy as the forging stock has an α=β microstructure,

[0030] the aspect ratio of primary α-phase is 5 or less,

[0031] the average grain size of primary α-phase is 10 μm or less, and

[0032] the volume fraction of primary α-phase is within a range from 20%or more to 80% or less,

[0033] wherein the aspect ratio is defined as the following ratio;

[0034] the aspect ratio=the longitudinal length of a grain/the width ofthe grain, which is perpendicular to the longitudinal direction

[0035] Seventhly, the present invention provides a forged titaniumalloy, which comprises 1.2 or less of work hardening factor defined byHv (def)/Hv (ini),

[0036] where Hv(ini) is the hardness of the titanium alloy as theforging stock before forging, and,

[0037] Hv(def) is the hardness of the forged titanium alloy under thereduction of 20% within a temperatures range from (Tβ−400)° C. or moreto less than 900° C., wherein the β-transus (° C.) of the titanium alloyis Tβ (° C).

[0038] Eighthly, the present invention provides the forged titaniumalloy according to the seventh material, wherein the difference ofhardness between a thickness center portion of the forged titanium alloyand near the surface area of the forged titanium alloy is 60 or less ofVickers hardness.

[0039] Ninthly, the present invention provides the forged titanium alloyaccording to the ninth material, consisting essentially of 4 to 5% Al,2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance ofsubstantially Ti.

[0040] Tenthly, the present invention provides the forged titanium alloyaccording to the seventh material, wherein

[0041] the titanium alloy as the forging stock has an α+βmicrostructure,

[0042] the aspect ratio of primary α-phase is 5 or less,

[0043] the average grain size of primary α-phase is 10 μm or less, and

[0044] the volume fraction of primary α-phase is within a range from 20%or more to 80% or less,

[0045] wherein the aspect ratio is defined as the following ratio;

[0046] the aspect ratio=the longitudinal length of a grain/the width ofthe grain, which is perpendicular to the longitudinal direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 is a graph showing the relationship between the heatingtemperature and surface oxidation in titanium alloys.

[0048]FIG. 2 is a graph showing the relationship between the averagegrain size of primary α-phase and the elongation.

[0049]FIG. 3 is a graph showing the relationship between the averagegrain size of primary α-phase and the fatigue strength.

[0050]FIG. 4 illustrates the forging method of Example 1.

[0051]FIG. 5 illustrates the forging method of Example 2.

[0052]FIG. 6 illustrates the forged shape of Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Concerning the detail of the present invention, the preferredembodiments have been described as follows.

[0054] The present invention has a specific technical feature that amechanism of a grain boundary sliding with diffusional accommodationduring being deformed at a given temperature are utilized efficiently,when a titanium alloy is forged. Some kinds of the titanium alloy havesuch a specific mechanism.

[0055] It is known that a large amount of deformation is attainable, dueto the grain boundary sliding with diffusional accommodation, under thecondition of the given temperature and under the condition of the givenstrain rate, when some kinds of titanium alloy are allplied to. In thiscase, the work hardening does not occur and a homogeneous microstructurecan be obtained in the forged titanium alloys.

[0056] In the usual forging method, it is easy to be off the propercondition, due to temperature drop of the work material and the frictionby contacting with the die in the conventional forging, even if theinitial conditions are fitted to the grain boundary sliding withdiffusional accommodation. From the standing point of view, which is,for solve the problems, the present invention provides that thetemperature of the work material and that of the die are defined astheir optimum range. And the present invention provides that thetitanium alloy is forged to get the optimum composition and the optimummicrostructure. Consequently, the forging method of the presentinvention could be found out such as an excellent workability, anexcellent material property and an excellent surface property.

[0057] The mechanism of the grain boundary sliding with the diffusionalaccommodation in the forging process can be verified, by way of thecomparison with hardness of work material between before and afterforging. As an ideal concept, when the mechanism of the grain boundarysliding with the diffusional accommodation works in the forging, pile up(accumulation) of dislocation (transfomation) does not occur. As aresult, the hardness does not increase by forging work. However, in thereal method, increase of hardness is unavoidable in the actual forging,due to the ununiform temperature of the work material. Taking theabove-mentioned facts into consideration, it is defined that themechanism of the grain boundary sliding with the diffusionalaccommodation is working in the forging, when HV(def)/Hv(ini) is 1.2 orless than 1.2 in this invention. Hv(ini) is the hardness of the titaniumalloy as the forging stock before forging, and Hv(def) is the hardnessof the forged titanium alloy under the reduction of 20% within atemperatures range from (Tβ−400)° C. or more to less than 900° C.,wherein the β-transus (° C.) of the titanium alloy is Tβ (° C.).Reduction ratio of actual forging is from 20% to 80% although it dependson the final shape. So it is defined that HV(def) is hardness of workmaterial forged at 20% of reduction ratio.

[0058] When the material is deformed under the mechanism of the grainboundary sliding with the diffusional accommodation, work hardening isslight. Consequently, the difference of the hardness between thethickness center portion of the work material and near the near thesurface area of the work material is small. Therefore, a uniform forgedmaterial can be obtained. Concretely speaking, there is no differenceabout the material properties on all of the portions, independent fromthe different located portions. If the value of the above-described workhardening factor is not more than 1.2, such a kind of the titanium alloyhas the material properties, concerning the difference in hardness of Hv60 or less between the surface layer and the inner portion. Thishardness prevents from generating the different material property amongeach portion such as ductility and fatigue strength. (Note: Herein-above and here in-after, near the surface area is defined as within arange of approximately 5 mm or less distant from the surface of thematerial after forged, although the distance depends on the size of theforged product.

[0059] The forging condition for getting the work hardening factor 1.2or less has been described, as follows.

[0060] According to the present invention, the forging is achieved on atitanium alloy, which has the β-transus of Tβ (° C.) at a strain rate,whose range is from 2×10⁻⁴ s⁻¹ to 1 s⁻¹, while keeping the relation of(Tβ−400)° C. ≦Tmβ≦900° C. and 400° C. ≦Td≦700° C. Here, Tm(° C.) isdefined as the temperature of the starting material for forging, andTd(° C.) is defined as the temperature of die.

[0061] At first, according to the present invention, it is required toexecute forging within the given temperature range and under the givencondition about the strain rate, to induce deformation. The deformationis caused by the grain boundary sliding with the diffusionalaccommodation. Generally speaking, concerning the titanium alloys, thetemperature range, which induces deformation caused by the grainboundary sliding with diffusive accommodation, is below the β-transus.Accordingly, the work material temperature Tm is required to be within atemperature range of below the β-transus.

[0062] If the forging temperature is below [Tβ−400(° C.)], the workhardening factor becomes excessively more than 1.2. When the titaniumgoes on being forged, there is one possibility to generate a largeamount of crack, even in case that the titanium has excellentworkability. Consequently, One kind of difficulty happens, that is, thedifficulty influences on producing the primary product and on thesecondary product. Additionally, while the titanium alloy is worked, thedeformation resistance increases remarkably. From the standing point ofthe capacity of the forging machine, it is not preferable to encounterwith the above-mentioned kind of difficulty.

[0063] On the other hand, in a high temperature range, the oxidationproceeds on to a great extent. Therefore, both from one aspect ofspending a lot of time on surface finishing of the forged titanium alloyafter forged, and from the other aspect of production-yields afterforged, it is an essential condition to forge the titanium alloy below900° C., in order to suppress the oxide layer to the degree of 100 μm orless. FIG. 1 shows the relationship between the heating temperature andthickness of oxidation layer of the titanium alloy. In case of thetitanium alloys, as seen in FIG. 1, the fact is found out that theoxidation on the surface of the titanium alloy increases rapidly, whenheating temperature is over 900° C. Within a temperature range less than900° C., the oxidation of the titanium alloy is suppressed. And thethickness of the oxidized layer invites a satisfactory result, which is,sufficiently less than 100 μm, by the reason of suppression. In casethat a temperature range for forging is adopted to be 870° C. or less,the thickness of the oxidation layer is suppressed, being decreased tothe degree of 50 ìm or less. In this way, the present invention makes itpossible to suppress the oxidation layer of the titanium alloy, more andmore.

[0064] Additionally speaking, determining the temperature of the die,Td(° C.), which is 400° C. or more, enables us to suppress thetemperature drop of work material by contact with die. And, dietemperature control makes it possible to prevent from deteriorating theworkability of the forged material. Simultaneously with theabove-mentioned results, the following good results are brought out.That is to say, a precise forgeability can be attained, and a crack canbe avoided. The precise forgeability and the avoidable crack areadaptable to everywhere. It goes without saying such as the parts, whichhave a thinner thickness. The higher the die temperature is, the biggerbecomes the suppressing temperature drop of work material. However, incase that the die temperature is higher than the β-transus, therehappens a problem that the temperature of the work material for beingforged has a possibility to rise up to the β-transus or more.Furthermore, even when at a temperature of the β-transus or less, andadditionally when the temperatures above 700° C., an expensive materialsuch as Ni-base alloy which has heat resistance and oxidationresistance, is required. So, this problem is not preferable, from theviewpoint of the cost-performance on forging. In addition, concerningthe die production, which is made of the above-mentioned material, anexpensive production method such as using an electric dischargemachining one is required. Higher reheating temperature makes the dieoxidize, and the temperature makes the tool oxidize, in addition to thecorresponding work material. Then, the oxidation forces the die and thetool to live on for a short life.

[0065] From the other technical standing point of view, that is, thedurability, which is mentioned above, it is not preferable to exceed thetemperature of 700° C.

[0066] In order to induce deformation caused by the grain boundarysliding with diffusional accommodation during forging, and in order tokeep work hardening factor of 1.2 or less, the following strain rate isnecessary, which is, within a range of from 2×10⁻⁴ s⁻¹ or more to 1 s⁻¹or less. Compared with a strain rate in a conventional forging process,a slightly slower strain rate is determined. And compared with a strainrate in an isothermal forging process, a faster strain rate isdetermined. That is to say, between 2×10⁻⁴ s⁻¹ or more to 1 s⁻¹ or less.This strain rate results in avoiding a long extended working time in theisothermal forging, and the strain rate results in achieving anefficient forging. In addition, the mechanism of the grain boundarysliding with the diffusive accommodation is made utilize of. As aresult, a favorable workability and a uniform microstructure afterforging are attained. The above-mentioned factor invites, also, theeffective result, that is, the material properties are much improved,such as ductility and fatigue properties.

[0067] Furthermore, taking the more chances to make utilize of themechanism of the grain boundary sliding with diffusional accommodationinto the consideration, the preferable strain rate in the forgingprocess is arranged within a range of from 1×10⁻³ s⁻¹ or more to 0.1 s⁻¹or less.

[0068] Furthermore, in order to keep the work hardening factor of 1.2 orless, and in order to keep the difference between the hardness of thethickness center portion of the work material and that of neighborhoodof the surface area of the work material, as Hv 60 or less, it ispreferable to execute the forging under the condition of, adding to theabove-given condition, keeping the relation of [(Tm−Td)≦250° C.] betweenthe die temperature, Td(° C.) and the temperature of the work materialfor being forged, Tm(° C.). The execution to forge under the relation of[(Tm−Td)≦250° C.] brings up the result of having improved themicrostructure difference between near the surface area, where thecooling speed is fast, and the thickness center portion, where thecooling speed is slow. Forged products with uniform material propertiescan be obtained by this way. If the temperature difference between thedie temperature, Td, and the temperature of the work material, Tm, ismore than 250° C., it is unfavorable, because the ununiform materialproperties in the forged product is likely to be generated, caused bythe temperature difference during forging between near the surface areaand the thickness center portion. In case of particularly large-sizedforging materials, it spends a longer time to forge the material. Andthe load for forging increases, too. From this standing point, it iseffective to control the temperature by the manner that the temperatureof the work material, Tm, and the temperature of the die, Td, come closeto each other, such as by the manner to satisfy the relation of[(Tm−Td)≦250° C.].

[0069] In the present invention, the titanium alloy, which is used asthe forging, stock preferably consists essentially of 4 to 5% Al, 2.5 to3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass percentage, and thebalance of substantially Ti. The term “balance of substantially Ti”referred herein is defined as a material, which contains inevitableimpurities and other trace-quantity elements, have a possibility toexist within the specified range showed in the present invention, unlessthese inevitable impurities and other trace-quantity elements cancel thefunction and the effect of the present invention.

[0070] Compared with conventional kinds of titanium alloy, the presentinvention allows the titanium alloy to deform, which causes by the grainboundary sliding with diffusional accommodation in a low temperaturerange from 700° C. to 870° C. Therefore, without thicker oxidationscale, without deteriorating the surface layer, and withoutdeteriorating the formation of α-case, the present invention enables thetitanium alloy to be forged. The reason is written up as follows, why itis indispensable to specify the composition of the titanium alloy.

[0071] Al is an essential element for an α+β type titanium alloy, inorder to stabilize the α-phase, and the Al has an effect on increasingthe strength. If the Al content is less than 4%, the AL content cannotcontribute to the degree of the sufficient material strength.

[0072] If the Al content exceeds 5%, the ductility and the toughnessdeteriorate. Both of the above-mentioned results, which mean, thematerial strength, the ductility and the toughness, are not preferable.

[0073] V, Mo, and Fe are elements, in order to stabilize the a phase andhave an effect to increase the strength. The V content, if less than2.5% cannot contribute sufficiently to high strength. In this case, theβ phase becomes unstable. On the contrary, if the V content exceeds3.5%, lowering the β-transus causes the problem to narrow the processingwindow, and furthermore, adding increase of cost due to addition of alarge amount of expensive alloying element.

[0074] Mo has an effect to refine microstructure and has an effect tosuppress the grain growth. Fe has high diffusibility in titanium. Withrespect to these effects, which are caused by Mo and Fe, the preciseforgeability increases. On the contrary, the hot deformation resistanceduring forging decreases. And the above-mentioned results bring up theadditional good effects, such as improving the ductility and the fatigueproperties after forging.

[0075] If the Mo content is less than 1.5%, a sufficient contribution tostrengthening cannot be obtained. And also, the β phase cannotsufficiently be stabilized. If the Mo content exceeds 2.5%, lowering theβ-transus causes to narrow the range of the processing window.Furthermore, effects of Mo and Fe are saturated by adding Mo and Fewithin the range of 2.5% or more, and by adding a large amount of anexpensive alloying element causes high cost. Supplementary speaking, theβ-phase becomes to be too stable. In this case, it is harmful forstrengthening by solution treatment and aging. If the Fe content is lessthan 1.5%, contribution of Fe to strengthening is not sufficient,simultaneously without the β-phase being unstable. Furthermore,regardless with one of the good factors about Fe, which means, Fe has acharacteristic to diffuse rapidly in titanium and to improve theworkability efficiently, the advantage of such characteristic, which Fehas, cannot be effective on the preferable results. Contrarily, if theFe content exceeds 2.5%, lowering the β-transus causes narrowing theprocessing window. Additionally speaking, segregation deteriorates thematerial properties. Furthermore, by specifying the alloy composition asdescribed above, the mutual quantity ratio of α-phase and β-phase isgetting to be closer to each other, within a temperature range of from700° C. to 870° C. It becomes easier to activate the mechanism of thegrain boundary sliding with additional accommodation.

[0076] According to the present invention, the titanium alloy, which isused as the forging stock, it is preferable that the microstructure is aα+β type, whose aspect ratio has 5 or less of primary α-phase, has 10 μmor less of the average grain size of primary α-phase, and has from therange of 20 or more to 80% or less, as a volume fraction of primary αphase, where the aspect ratio is defined as the ratio of the following:

[0077] a) Longitudinal length of a grain To

[0078] b) Width of the grain, which is perpendicular to the longitudinaldirection thereof.

[0079] That's to say, a)/b).

[0080] More preferably, the titanium alloy has 6 μm or less of theaverage grain size of the primary α-phase.

[0081]FIG. 2 is a graph showing the relationship between the averagegrain size of the primary α-phase and the elongation. As shown in theFIG. 2, if the average grain size of the primary α-phase exceeds 10 μm,the elongation in the tensile test at high temperature decreasesrapidly, whose phenomenon influences on the sensitivity for cracking andon the precise forgeability and the like.

[0082] Furthermore, the grain size of primary α-phase influences on thematerial properties of the forged product, such as the ductility and thefatigue properties. FIG. 3 shows the relationship between the averagegrain size of primary α-phase and the fatigue properties. As shown inthe FIG. 3, if the average grain size exceeds 10 μm, the sensitivity forcracking during forging increases, and the precise forgeabilitydeteriorates, additional to a result that the material properties suchas the ductility and the fatigue properties deteriorate.

[0083] The shape of the primary α-phase influences on the sensitivityfor cracking and the precise forgeability. When the aspect ratio isdefined as the ratio of longitudinal length of a grain to width thereofperpendicular to the longitudinal direction thereof, such as mentionedabove, and in case that the aspect ratio of primary α-phase exceeds thevalue of 5, the primary α-phase cannot become into the equiaxed grain.Consequently, the precise forgeability deteriorates.

[0084] Furthermore, fine equiaxed microstructure improves susceptibilityto cracking in the hot forging, suppresses the cracking duringdeformation at high strain rate and improves precious forgeability. Anα+β type titanium alloy, generally, consists of primary α-phase andtransformed β-phase. However, in case that the volume fraction of thetransformed β-phase becomes to come within a range of from 20 or more to80% or less, that's to say, in case that the volume fraction of theprimary α-phase becomes to less than 20% or more than 80%, thesensitivity for cracking during forging increases, too. Not only theproblem of the cracking, but the precise forgeability, the ductility andthe fatigue properties of the material deteriorates.

[0085] In the present invention, even after forging, the forged productcan have the microstructure, which is similar to that of the forgingstock. The way means making use of the grain boundary sliding withdiffusional accommodation. Owing to such advantageous characteristics,the present invention is extremely effective on improving theworkability and the material properties, even in case of repeating theforging, and even in case of applying such forging process to theworking for a complex forged shape.

EMBODIMENTS

[0086] In order to explain the above-described effective functions,there has been described the effects of forging conditions of titaniumalloy, the chemical composition of the forging stock. Furthermore, theeffects of the microstructure on the forgeability and the materialproperties after forging have been described to the examples.

EXAMPLE 1

[0087] Cylindrical compression test samples, whose size are 15 mm indiameter and 22.5 mm in height, were cut from material “A01” as shown inTable 1. The sample was forged at reduction of 20% using a die made bySUS310, while varying the forging temperature, the die temperature, andthe strain rate. Table 2 shows the forging conditions, the workhardening factor [Hv(def)/Hv(ini)], and the difference in hardnessbetween near the surface area and the thickness center portion. Thetemperature of the work material, Td, in the formula (Tm−Td), was takeninto consideration of temperature range from starting and finishing offorging.

[0088] Nos. 1 to 3 samples were forged under the conditions of theforging temperature, the die temperature, and the strain rate, whosevalues satisfy the conditions of the present invention. And the resultsinvited a value of 1.2 or less as work hardening factor, and 60 or lessas the difference of Vickers hardness between near the surface area andthe thickness center portion. Consequently, the hot-forging processunder the conditions of the present invention induces the deformationcaused by the grain boundary sliding with diffusional accommodation. Andthe hot-forging in the present invention brings up an excellent resultsthat uniform and homogeneous forged products can be obtained, whichmeans, there is no difference in the each part of the forged material.

[0089] On the contrary, titanium alloys, which were forged under theconditions out of this invention, showed a large work hardening factor,more than 1.2, and showed 60 or more as the differential value in thehardness between near the surface area and the thickness center portion.TABLE 1 Microstructure of forging stock Volume fraction of Alloyβ-transus Average grain primary α phase Aspect Symbol (numeral is mass%) (° C.) size(μm) (%) ratio A01 Ti-4.5Al-3.2V-2.2Fe-1.8Mo 900 2.6 274.4 A02 Ti-4.5Al-3.2V-2.2Fe-1.8Mo 900 3.7 30 1.8 A03Ti-4.5Al-3.2V-2.2Fe-1.8Mo 900 3.0 48 1.1 A04 Ti-4.5Al-3.2V-2.2Fe-1.8Mo900 5.0 45 1.3 B01 Ti-6.1Al-3.9V 1000 8.3 85 4.7 B02 Ti-6.1Al-3.9V 10005.9 80 1.9 B03 Ti-6.1Al-3.9V 1000 6.4 83 6.9 B04 Ti-6.1Al-3.9V 1000 12.982 1.7 B05 Ti-6.1Al-3.9V 1000 10.3 79 1.1 B06 Ti-6.1Al-3.9V 1000 — 0 —C01 Ti-10.2V-2.2Fe-2.9A1 800 5.1 10 1.1 D01 Ti-6.1Al-1.9Sn-1.8Zr-1.9Mo-960 3.0 50 1.1 2.0Cr

[0090] TABLE 2 Work material temperature (° C.) Die Work Reheating-Starting Finishing temperature Strain rate Hv(def)/ Hv hardness Symbolmaterial temperature temperature temperature (° C.) (s⁻¹) Tm-Td (° C.)Hv(ini) difference 1 A01 830 800 750 620 to 600 0.051 ≦180 1.05 30 2 A01830 800 750 700 to 670 0.004 ≦100 1.02 8 3 A01 830 800 750 600 to 5800.085 ≦200 1.07 21 4 A01 950 920 850 600 to 580 0.085 ≦320 1.22 68 5 A01830 800 750 620 to 600 1.1 ≦180 1.29 85 6 A01 830 800 750 300 to 2900.037 ≦500 1.33 101 7 A01 850 830 800 550 to 530 0.037 ≦280 1.24 74

EXAMPLE 2

[0091] Using the cylindrical compression samples, which have 15 mm indiameter and 22.5 mm in height, which have the chemical compositions andmicrostructures given in Table 1, hot forging was performed, as shown inFIG. 4. The hot forging was conducted, under as the same condition asTable 3, using a die of SUS310 and without lubricant. The workability,the condition of the oxidation surface, and the microstructure afterforging at near the surface area on a protruded section and at thethickness center portion of the disk shaped section on the lower part,were evaluated. The results are given in Table 3. In Table 3, the mark“∘” in the column “Crack” indicates “no crack occurred”, and the mark“X” in the column indicates “crack occurred”. Nos. 1, 13, and 24 inTable 3 has βmicrostructure, so the average grain size of the primary αphase and the aspect ratio were not measured. TABLE 3 Reheating StartingFinishing Die Strain Work temp. temp. temp. temp. rate T H Symbolmaterial (° C.) (° C.) (° C.) (° C.) (s⁻¹) (mm) (mm) H/T Crack 1 A01 950920 850 600-580 0.051 9.30 11.00 1.18 X 2 A01 830 800 780 700-670 0.0455.90 12.30 2.08 ◯ 3 A01 830 800 750 600-580 0.042 6.05 12.20 2.00 ◯ 4A01 830 800 750 600-580 0.086 6.15 12.30 2.00 ◯ 5 A01 830 800 750600-580 1.1 7.50 11.20 1.49 ◯ 6 A01 830 800 600 300-290 0.037 7.50 10.301.37 X 7 A01 800 780 730 600-580 0.0069 6.10 12.20 2.00 ◯ 8 A01 650 620440 400-380 0.027 10.10 12.40 1.23 X 9 A02 830 800 750 600-580 0.0436.15 12.30 2.00 ◯ 10 A03 830 800 750 600-580 0.0044 6.05 12.20 2.02 ◯ 11A03 830 800 750 650-580 0.00036 6.05 12.45 2.06 ◯ 12 A04 830 800 750600-580 0.043 6.20 12.40 2.00 ◯ 13 B01 1050 970 880 600-580 0.027 10.1010.90 1.08 X 14 B01  950 920 860 700-670 0.039 7.00 12.00 1.71 ◯ 15 B01 950 900 820 600-580 0.038 7.20 12.00 1.67 ◯ 16 B01 950 900 820 600-5800.075 7.30 11.90 1.63 ◯ 17 B01 950 900 820 600-580 1.9 8.50 11.10 1.31 ◯18 B01 950 900 700 300-290 0.032 8.50 10.20 1.20 ◯ 19 B01 650 620 440400-380 0.023 11.20 12.30 1.10 X 20 B02 950 900 820 600-580 0.04 6.8012.25 1.80 ◯ 21 B03 950 900 820 600-580 0.039 7.00 12.00 1.71 ◯ 22 B04950 900 820 600-580 0.039 6.90 12.05 1.74 ◯ 23 B05 880 850 780 600-5800.039 8.50 11.10 1.31 ◯ 24 B06 950 900 820 600-580 0.027 10.00 11.001.10 X 25 C01 750 720 540 500-400 0.042 6.45 11.80 1.83 ◯ 26 D01 900 850720 600-580 0.042 6.35 12.10 1.91 ◯ Thickness Thickness center portionSurface layer portion of Average Volume Average Volume Surface oxidationgrain fraction grain fraction condition layer size of primary Aspectsize of primary Aspect after Symbol (μm) (μm) α phase (%) ratio (μm) αphase (%) ratio forging 1 150 — 0 — — 0 — A 2 40 3.5 30 1.3 2.6 26 2.3NA 3 40 3.3 38 1.4 2.7 25 3.3 NA 4 40 2.9 40 1.3 2.6 28 2.6 NA 5 40 2.637 1.1 2.5 28 2.9 NA 6 40 2.9 38 1.5 2.3 27 6.3 A 7 35 2.9 39 1.5 2.5 282.3 NA 8 20 2.6 28 2.3 2.2 27 7.2 A 9 40 4.2 41 1.3 3.5 31 2.3 NA 10 404.6 39 1.1 3.1 49 1.1 NA 11 40 5.8 42 1.1 4.8 48 1.1 NA 12 40 5.7 40 1.14.6 46 1.2 NA 13 350 — 0 — — 0 — A 14 150 8.3 84 3.7 13.4 79 5.2 A 15150 8.1 83 2.6 15.4 77 5.3 A 16 150 8.0 84 2.7 12.9 73 5.3 A 17 150 7.985 3.0 12.7 73 5.5 A 18 150 7.3 83 6.4 14.1 80 6.5 A 19 20 7.1 85 7.314.5 71 6.5 A 20 150 5.7 85 1.3 12.1 77 5.5 A 21 150 5.9 83 3.1 13.0 755.3 A 22 150 11.1 80 1.6 15.4 78 5.3 A 23 60 9.4 81 1.1 15.7 78 6.7 A 24150 — 0 — — 0 — A 25 30 4.9 11 1.1 11.1 8 5.2 A 26 70 2.8 48 1.1 5.5 445.2 A

[0092] The microstructure of the forging stock and the microstructure ofthe forged product was evaluated by the average grain size of primaryα-phase, the volume fraction of the primary α-phase, and the aspectratio. The forgeability was evaluated by the precise forgeability in theactual forged result, and by the sensitivity for cracking, mainly byobserving the surface condition of the forged product. The preciseforgeability was evaluated by such a way as the comparison of theprotrusion height, that's to say, how much degree of metal fills existedin the circular holes in the die with metal. (See FIG. 4). That is, asillustrated in FIG. 4, the height including the height of the spike likeshape protrusion, was defined as H. And the thickness of the diskportion was defined as T. Finally, the precise forgeability wasevaluated by the ratio of the value H/T. In order to attain thefavorable forgeability, the value of H/T needs 1.5 or more, preferablyneeds 2.0 or more. Moreover, in order to evaluate the results,concerning to how much degree the surface was finished after thematerial was forged, the thickness of the layer (oxidation layer), whichwas caused by the oxidation at the surface layer portion of the forgedproduct, was measured.

[0093] With regard to the Nos. 1 and 13, whose forging temperatures wereabove the β-transus, cracks were observed. And the parameter value H/T,which evaluated the precise forgeability, was as small as around 1.2.From the judging point of the precise forgeability, it was poor. Withrespect to the Nos.1, from 13 through 18, and from 20 through 22, whoseforging temperatures were above 900° C., the thickness value of theoxidation layer exceeded 100 μm. Concerning the Nos. 8 and 19, whoseforging temperatures were low, cracks were observed. Furthermore, theH/T value was as small as around 1.2, resulting in the poor preciseforgeability.

[0094] Thinking about the Nos. 6, 8, 18, and 19, whose die temperaturewere fallen outside of the range of the present invention, the parametervalue H/T was as small as 1.5 or less. In a few cases, no crackhappened, but in a lot of cases, they have inferior preciseforgeability.

[0095] With regard to the Nos. 5 and 17, whose strain rate were fallenoutside of the range of the present invention, the parameter H/T, whichevaluated the precise forgeability, were smaller than 1.5. In a fewcases, no cracks were observed. But, looking at the results in a lot ofcases, there was inferior quality, from the standing point of theprecise forgeability.

[0096] As described above, in case that the conditions were fallenoutside of the range of the present invention, cracks were observed andprecise forgeability was deteriorated. In this case, it could be foundout that there generated no deformation, which was caused by the grainboundary sliding with diffusional accommodation.

[0097] Secondary, the effects of chemical composition of forging stock,average grain size, volume fraction and aspect ratio of primary α in theforging stock on the forgeability were studied.

[0098] From the viewpoint of the range of the composition in the presentinvention, Nos. A01 to A04 is satisfactory, and their microstructure iswithin the range of the present invention. No cracks were observed inNos. from 2 to 4, from 7, and from 9 to 12. When the forging goes on,the conditions of the present invention are, absolutely, necessary, inorder to obtain the good results. Additionally speaking, theabove-mentioned case showed a very excellent forgeability, which caneasily be understood by the extremely high value of H/T≧2, as in thecases of Nos. from 2 to 4, 7, and from 9 to 12. Moreover, whichever theobjective portion is, for instance, the thickness center portion afterforging, or near the surface area after, the result ended up in the samemicrostructure. In this case, the same microstructure means that theforging stock has 10 μm or less of the average grain size of primaryα-phase, 20 to 80% of volume fraction, and 5 or less of aspect ratio.Furthermore, it means, that no remarkable difference in themicrostructure appeared between the thickness center portion and thenear surface area. Consequently, the fine microstructure, such that norough surface could generate, was obtained even on the near surfacearea.

[0099] In case that the forging stock, whose Nos. are from B01 to B06,C01, and D01, and whose chemical compositions are fallen outside of therange of the present invention, are made use of, the following resultswere shown. That is, in the Nos. 16, from 20 to 22, 25, and 26, exceptfor the temperature of forging stock, the materials were worked, underthe control of the same forging conditions as the present invention. Inthis case, it showed a resultant value of the 1.6 to 1.9 as H/T, whichis 1.5 or more, and which is a criteria regarding whether the preciseforgeability can be done or not. However, compared with the values ofH/T≧2.0 which is attained by using the forging stock according to thepresent invention, the value of H/T is not satisfactory, and it wasrevealed that the chemical composition and the microstructure of theforging stock, also, influence on the forgeability. Among these Nos.,the Nos. 20 and 26, which used the materials of B02 and D01,respectively, and which satisfied the range of microstructure of thepresent invention, showed a high H/T value, 1.80 and 1.91, respectively.However, the microstructure after forging, was fallen outside from therange of the present invention. As a resultant problem, a rough surfacehappened. Not only about the above-mentioned Nos. but also about theNos. 23 and 25, the microstructure after forging was fallen outside fromthe range in the present invention. In this case, the same problem, therough surface happened.

[0100] With regard to the No. 23, the chemical composition and themicrostructure went outside from the range in the present invention.Additionally speaking, the forging temperature was lower than the valueof the Nos. 16, and from 20 to 22. Although these values were within therange of the present invention, the H/T value was 1.5 or less. Moresupplementary, the No. 24, which used B06 having the β-microstructure,cracks were observed, and the H/T value was low.

[0101] Since the β-transus of the materials from B01 to B06 was as highas 1,000° C., these materials were possible to be forged in a hightemperature range, because the hot deformation resistance was small inthe high temperature range. But, such a high temperature forgingincreases a amount of the oxidized layer to be formed. According to theexamples in the present invention, the materials B01 through B04, andB06 adopted 950° C. as the reheating temperature, and the startingtemperature for forging was 900° C. Compared with the case of A01through A04, which had 900° C. of β-transus, the materials B01 throughB04, and B06 adopted higher forging temperature. So, the thickness ofoxidation layer became as thick as 150 βm.

[0102] With regard to the material, B05, which adopted 880° C. ofreheating temperature, in order to suppress the oxidization, and whosestarting temperature for forging was 850° C., the low temperaturedeteriorated the forgeability to invite 1.5 or less of H/T value,although the thickness of the oxidation layer decreased. Furthermore,these examples that had compositions, which were not satisfying therange of the present invention, invited the result that there weredifference about the microstructure, between near the surface area andthe thickness center portion after forging. And there generated therough surface, caused by the coarse grains and the elongated grainstructure.

EXAMPLE 3

[0103] Using the forging stock, A01 and B01 given in Table 1, whose sizewas 30 mm in width, 60 mm in height, and 70 mm in length, thehot-forging illustrated in FIG. 5 was conducted, under the conditions inaccordance with Table 4. The resultant forged products had the size ofapproximately 30 mm in width, 20 mm in height, and 210 mm in length.From each of the forged products, samples were cut and prepared. Themechanical properties of these samples are evaluated from the judgingpoints of the Vickers hardness, the tensile properties, and the fatigueproperties of the flat plate test piece. The results are shown in Table4. TABLE 4 Die Reheating Starting Finishing temp. Strain Tensileproperties Fatigue Work temp. temp. temp. range rate 0.2% PS UTS Elstrength Symbol material (° C.) (° C.) (° C.) (° C.) (s⁻¹)Position(location) Δ HV (MPa) (MPa) (%) (MPa) 1 A01 830 800 750 600 to0.042 Near the surface layer 10 988 1040 16 850 580 Thickness center 9801030 18 850 2 B01 950 900 820 450 to 0.043 Near the surface layer 651017 1070 9 640 430 Thickness center 961 1010 14 550

[0104] The No. 1, which satisfied the temperature of the material forbeing forged, the temperature of the die, and the strain rate, accordingto the present invention, invited the difference (ΔHv) of 60 or less asthe Vickers hardness between two portions. That is, one portion meansnear the surface area, where the temperature drop by contact with die issignificant. The other portion means the thickness centers portion,where the cooling speed is comparatively slow. In this case, thedifference (ΔHv) is 60 or less value, whose value is in accordance witha recommendable condition in the present invention. Changing theviewpoint from the tensile properties and the fatigue properties, thedifference between these portions became smaller. The result brought upa excellent and possible method to produce the forged product that has auniform and a homogeneous material properties. On the other hand, theNo. 2, which was forged under the forging condition fallen outside fromthe range of present invention, invited 60 or more of ΔHv. In case ofthe No. 2, the difference in hardness happened, between near the surfacearea and the thickness center portion. And more kinds of the differencehappened, that are, the material properties such as the static strength,the ductility, and the fatigue strength between these portions. Theresult is not preferable, from the standing point of the uniform andhomogeneous material properties. As described above, it can be found outdefinitely that the forging conditions of the present invention areextremely important, from the high technological viewpoint of producingthe forged product, which has a uniform and a homogeneous forgedmaterial.

EXAMPLE 4

[0105] Using the forging stock, which was the No. A01 given in Table 1,and whose size was 150 mm in diameter and 750 mm in length, the hotforging was adopted, in order to obtain a shape shown in FIG. 6. Thehot-forging was conducted under the condition of 800° C. of heatingtemperature of the forging stock, 780° C. of the starting temperature offorging, 670° C. of the finishing temperature of forging, the dietemperature range within from 650° C. to 620° C. during forging, and2.3×10⁻³ of the strain rate. In this case, the forgeability regarding alarge-sized forged product was evaluated. Adaptable samples were cut andprepared from the forged shape at each position given in FIG. 6.

[0106] And the tensile strength as the material properties wasevaluated. Furthermore, the fatigue strength as the material properties,while using the specimen that was prepared by the rotation-bending, testwas evaluated. The results are shown in Table 5. TABLE 5 Tensileproperties Fatigue Average Volume fraction 0.2% PS UTS El RA strengthgrain size of primary α- Aspect Position ΔHV (MPa) (MPa) (%) (%) (MPA)(μm) phase (%) ratio Near surface area 10 948 998 17 51 620 3.9 38 1.1Thickness center 940 989 19 54 610 4.6 42 1.0

[0107] By making use of a forging stock, which has a chemicalcomposition and a microstructure that satisfy the conditions of thepresent invention, it was found out that forging large-sized member ofthe titanium alloys can be attained. And, even when such forging isadapted to the titanium alloys, which has a difficulty to be worked asthe property, the same attainable results could be found out. And in thepresent invention, it was found out that the material propertiescorresponding to the obtained forged product are extremely favorable.

[0108] The Effectiveness of the Present Invention

[0109] As described above, the present invention makes it easilypossible to provide a high strength forged product from the titaniumalloy. The characteristics of the high strength forged product of thetitanium alloy have a narrow distribution of the material properties,towards the thickness direction. This invention make it easily possibleto remove the oxidation layer and the invention make it possible tofinish the surface of the forged product, after forging, during beingworked in order to obtain the final figure and shape. Furthermore, theinvention makes it easily possible to obtain a less sensitivity forcracking, possible to obtain an excellent workability of the forgedtitanium alloy, a good quality about the ductility and about the fatiguestrength. Finally, the present invention invites an excellent and a fineforged titanium alloy, whose strength is extremely high. Thus, thepresent invention has a big deal of effectiveness on the industrial andthe applicable usage.

What is claim is:
 1. A method for forging a titanium alloy comprisingthe steps of: preparing a titanium alloy as a forging stock, wherein thetitanium alloy has a thickness center portion and near the surface area;forging the titanium alloy as the forging stock to have a work hardeningfactor, whose value is 1.2 or less, for obtaining a forged titaniumalloy having a uniform and homogeneous material properties, wherein thework hardening factor is defined as, work hardeningfactor=Hv(def)/Hv(ini) where, Hv(ini) is the hardness of the titaniumalloy as the forging stock before forging, and, Hv (def) is the hardnessof the forged titanium alloy under the reduction of 20%.
 2. The methodaccording to claim 1, wherein a difference of hardness between thethickness centers portion of the forged titanium alloy and near thesurface area of the forged titanium alloy is 60 or less of Vickershardness.
 3. A method for forging a titanium alloy comprising the stepsof: preparing a titanium alloy as a forging stock; forging the titaniumalloy as the forging stock, at a strain rate within a range from 2×10⁻⁴s⁻¹ or more to 1 s⁻¹ or less, while keeping a relation of (Tβ−400)°C.≦Tm≦900° C. and 400° C.≦Td≦700° C., to obtain a forged titanium alloyhaving a uniform and homogeneous material properties; where, Tβ (° C.)is a β-transus of the titanium alloy, Tm(° C.) is a temperature of thework material for being forged, and Td(° C.) is a temperature of a die.4. The method according to claim 3, wherein the temperature of the die,Td(° C.), and the temperature of forging stock, Tm (° C.), arecontrolled to satisfy the relation of (Tm−Td)≦250° C.
 5. The methodaccording to claim 3, wherein the titanium alloy as the work materialfor being forged contains Al: 4 to 5%, V:2.5 to 3.5%, Fe:1.5 to 2.5%,and Mo:1.5 to 2.5%, by mass percentage.
 6. The method according to claim3, wherein, the titanium alloy as the forging stock has an α+βmicrostructure, an aspect ratio of primary α-phase is 5 or less, anaverage grain size of primary α-phase is 10 μm or less, and a volumefraction of primary α-phase is within a range from 20% or more to 80% orless, wherein the aspect ratio is defined as the following, the aspectratio=longitudinal length of the grain/width of the grain, which isperpendicular to the longitudinal direction.
 7. A forged titanium alloycomprising: a thickness center portion and near the surface area; a workhardness factor of 1.2 or less, wherein the work hardening factor isdefined by Hv(def)/Hv(ini), where, Hv (ini) is the hardness of thetitanium alloy as the forging stock before forging, and, Hv (def) is thehardness of the forged titanium alloy under the reduction of 20%,wherein the forging stock within a temperatures range from (Tβ−400)° C.or more to less than 900° C., and Tβ (° C.) is a β-transus (° C.) of thetitanium alloy.
 8. The forged titanium alloy according to claim 7,wherein difference of the hardness between the thickness center portionof the forged titanium alloy and near the surface area of the forgedtitanium alloy is 60 or less of Vickers hardness.
 9. The forged titaniumalloy according to claim 7 consisting essentially of 4 to 5% Al, 2.5 to3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance ofsubstantially Ti.
 10. The forged titanium alloy according to claim 7,wherein, the titanium alloy as a before forging has an α+βmicrostructure, an aspect ratio of primary α-phase is 5 or less, anaverage grain size of primary α-phase is 10 μm or less, and a fractionof primary α-phase is within a range from 20% or more to 80% or less,wherein the aspect ratio is defined as the following; the aspectratio=longitudinal length of the grain/length of the grain, which isperpendicular to the longitudinal direction.